Process control systems require the accurate measurement of process variables. Typically, a primary element senses the value of a process variable and a transmitter develops an output having a value that varies as a function of the process variable. For example, a level transmitter includes a primary element for sensing level and a circuit for developing an electrical signal proportional to sensed level.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advances in micropower impulse radar (MIR), also known as ultra-wideband (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and lost cost time domain reflectometry (TDR) instruments.
In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a probe that serves as a transmission line, in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. These instruments are also known as guided wave radar (GWR) measurement instruments.
One type of probe used by GWR level instruments is the coaxial probe. The coaxial probe consists of an outer tube and an inner conductor. When a coaxial probe is immersed in the liquid to be measured, there is a section of constant impedance, generally air, above the liquid surface. An impedance discontinuity is created at the level surface due to the change in dielectric constant of the liquid versus air at this point. When the GWR signal encounters any impedance discontinuity in the transmission line, part of the signal is reflected back toward the source in accordance with theory based on Maxwell's laws. The GWR instrument measures the time of flight of the electrical signal to, and back from, this reflecting point, being the liquid surface, to find the liquid level.
Simple level measurement involves detecting the reflected signal from a single level surface, such as water or oil. A slightly more complex measurement is so-called “interface” measurement, in which a less dense medium such as oil floats on top of a heavier medium such as water.
In many industrial processes, such as crude oil processing, tanks may contain both oil and water. However, the boundary between the oil and water may be poorly defined due to mixing of oil and water at the boundary. Instead of being sharp and well-defined, a layer of variable, potentially large, thickness consisting of an oil/water mix may exist. This “emulsion” layer typically starts out as a high percentage of oil mixed with a small percentage of water near the top. The percentage of water in the mix generally increases until the percentage of water in the mix is high and the percentage of oil low near the bottom of the emulsion layer, eventually becoming all water and no oil. Advantageously, it is desired to profile the emulsion as by measuring the oil/water mix inside the emulsion.
GWR is a good technology for so-called “interface” applications. However, conventional GWR struggles when an “emulsion” layer (a layer consisting of a mixture of oil and water) is present, because the boundary between the oil and water is no longer sharp or well-defined. The emulsion layer is comprised of a varying percentage of oil/water, and may be due to turbulence, mixing, etc., or simply being pumped from the ground that way. Emulsions can happen very frequently in oil-water processing at industrial plants (for example, crude oil processing). The emulsion layer thickness in these applications can range from an inch or less to nearly the entire height of the tank (40 feet or more).
While GWR provides a very desirable solution to simple level measurement and “clean interface” applications, GWR solutions to the emulsion measurement problem have been elusive. Difficulties arise in producing a measurable signal with conventional GWR when no distinct impedance change exists in the process due to the presence of the emulsion layer.
There would be significant commercial benefit to an emulsion measurement device that would operate in the same manner as, and have the benefits of, the well-known guided wave principles. The costs of known solutions to the emulsion measurement problem are very large (a nuclear-technology based emulsion measurement device can approach $100,000 in instrument cost). Another device promoted for use with emulsions is not only expensive but is simply a point-level sensor, incapable of “profiling” or continuous emulsion measurement. A “profiling” system is one in which the oil/water percentage in the emulsion can be detected and reported with good resolution over the length of the probe (and therefore the entire height of the tank or process).
This application is directed to improvements in emulsion measurement and profiling.